Mems controlled oscillator

ABSTRACT

An array of micromechanical oscillators have different resonant frequencies based on their geometries. In one embodiment, a micromechanical oscillator has a resonant frequency defined by an effective spring constant that is modified by application of heat. In one embodiment, the oscillator is disc of material supported by a pillar of much smaller diameter than the disc. The periphery of the disc is heated to modify the resonant frequency (or equivalently the spring constant or stiffness) of the disc. Continuous control of the output phase and frequency may be achieved when the oscillator becomes synchronized with an imposed sinusoidal force of close frequency. The oscillator frequency can be detuned to produce an easily controlled phase differential between the injected signal and the oscillator feedback. A phased array radar may be produced using independent phase controllable oscillators.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/598,097, Filed Nov. 9, 2006 which claims priority to U.S. ProvisionalApplication Ser. No. 60/734,836, filed Nov. 9, 2005 (entitled MEMSCONTROLLED OSCILLATOR). This application is also related to U.S. patentapplication Ser. No. 10/097,178 (entitled HEAT PUMPED PARAMETRIC MEMSDEVICE, filed Mar. 12, 2002) which are incorporated herein by reference.

GOVERNMENT FUNDING

The invention described herein was made with U.S. Government supportunder Grant Number DMR-0079992 awarded by National Science Foundation.The United States Government has certain rights in the invention.

BACKGROUND

Frequency entrainment is an interesting phenomena in nonlinearvibrations. It was discovered in the 17^(th) century by ChristianHuygens, who remarked that two slightly out of step pendulum-like clocksbecame synchronized after they were attached to the same thin woodenboard. Similar phenomenon, injection locking, was observed in radiofrequency (RF) circuits and laser systems.

Entrainment of micromechanical device oscillation has also been achievedin the RF range. Oscillation induced by a continuous wave laser can beentrained to a frequency near the resonant frequency of themicromechanical device by applying a small pilot signal at a frequencyclose to the resonant frequency, or by modifying the effective springconstant of the resonator by imposing a small RF component on the laserbeam intensity. Such oscillations may be difficult to precisely control.

When the mechanical oscillator is synchronized to the pilot signal, thevariations in the frequency and phase of the mechanical oscillations arelocked to and controlled by the frequency of the pilot oscillator. Suchan implementation offers the possibility of phase tuning or modulatingthe feedback of the mechanical resonance, however it suffers from thelack of a constant RF output frequency which is a requirement for phasedarray radar or phase modulated communication systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a MEMS structure for use as a heat pumpedparametric oscillator according to an example embodiment.

FIGS. 2A, 2B, and 2C illustrate a process of forming the MEMS structureof FIG. 1 according to an example embodiment.

FIG. 3 is a block schematic diagram of a MEMS resonator having a pilotsignal for entraining the resonator according to an example embodiment.

FIG. 4 is a block schematic diagram of a MEMS resonator with entrainmentand an adjustable resonant frequency according to an example embodiment.

FIG. 5 is a graph depicting phase shift versus bias modifying theresonant frequency of a resonator according to an example embodiment.

FIG. 6 is a block diagram of a phased array radar system with phasemodifiable emitters according to an example embodiment.

FIGS. 7A, 7B, 7C and 7D are graphical examples of arrays of emitterswith different controlled phase changes over the array to shape andcontrol beams according to an example embodiment.

FIG. 8 is a block diagram of a system 800 for modulating the phase orfrequency of a micromechanical reference oscillator according to anexample embodiment.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description is, therefore, not to betaken in a limited sense, and the scope of the present invention isdefined by the appended claims.

One example microelectromechanical (MEMS) device is first described,followed by a mechanism for entraining oscillations of a MEMS device toa pilot frequency. Methods and devices for controlling the frequency andphase of oscillations is then described, followed by examples ofapplications with devices having independently controllable frequencyand phase.

In one embodiment, continuous control of the output phase and frequencyof a micromechanical oscillator may be achieved when the oscillatorbecomes synchronized with an imposed sinusoidal force of closefrequency. The oscillator frequency can be detuned to produce an easilycontrolled phase differential between the injected signal and theoscillator feedback.

By controlling the DC bias of a self-generating, injection-locked,thermally actuated micromechanical resonator the frequency and phase ofthe output signal may be continuously varied. An array of suchsmall-sized tunable oscillators provide a compact device forimplementing beam steering or beam formation in a phased-array antenna.The ability to produce continuous frequency or phase variations may alsobe used for applications in wireless communications, where informationis encoded in phase or frequency modulated signals. By modulating a DCbias with a baseband signal, the MEMS oscillator can generate a widebandwidth phase or frequency modulated carrier frequency.

FIG. 1 is a side view representation of a micromechanical oscillator100. It oscillates in the radio frequency (RF) range and is fabricatedin the form of a silicon disc 110 supported by a SiO₂ pillar 120 at thedisc center. Other shapes, such as oval or polygons may also be used,and are included in the use of the term disc. An effective springconstant of this oscillator 100 is controlled within the range) f/f˜10⁻⁴by a low power laser beam (P_(laser)100:W) 125, focused at the periphery130 of the disc.

Oscillators in the RF range may be obtained. Generally, the smaller thestructure, the higher the frequency of oscillation. Modulating theintensity of the focused laser beam 125 provides changes of theeffective spring constant of the oscillator. The dc component of thelaser beam may be used to detect the vibration (at the fundamentalfrequency) by interferometric effects.

In still a further embodiment, a bridge structure is used as anoscillator. MEMS bridge structures are well known in the art. In thisembodiment, an AC current may be run through the bridge to vary itseffective stiffness.

As illustrated in FIG. 2A, commercially available silicon-on-insulator(SOI) wafers 210 with a 250 nm thick silicon layer 215 on top of a 1micron silicon oxide layer 220 may be used in one embodiment formicrofabrication. Other thicknesses of the layers may be used in variousembodiments to produce oscillators that have different resonantfrequencies. Discs of radius R from 5 to 20 microns may be defined byelectron-beam lithography followed by a dry etch through the top siliconlayer as shown in FIG. 2B. The radius of the discs also affects theresonant frequency. Dipping the resulting structure into hydrofluoricacid undercuts the silicon oxide starting from the disc's peripherytoward the center as shown in FIG. 2C. By timing this wet etch, thediameter of the remaining column of the silicon oxide 230, whichsupports the released silicon disc 235, is varied. In one embodiment,the diameter of the column is approximately 6.7 microns.

Oscillator 100 is just one example of oscillators that may be used withthe present invention. Other types of oscillators, such as cantileveredbeams, and bridges may also be used. Other types of mechanicaloscillators having resonant frequencies in desired ranges, and somedegree of nonlinearity for entrainment may be used.

In one embodiment, a 30 μm diameter polysilicon shell type resonatorwith resonant modes between 10 and 30 MHz and a mechanical qualityfactor of ˜10,000 may be used. The shell resonator is a circular, 200 nmthick plate, clamped on the periphery and suspended via removal of thesacrificial oxide in the center. Due to compressive stress in thepolysilicon film, the circular plate has a convex or concave dome-likecurvature, enhancing the resonant frequency of the device and providinga means of thermal actuation.

FIG. 3 is a block diagram showing entrainment of a resonator devicegenerally at 300. A resonator 310 is coupled to an input signal 315 toinitiate oscillation of the resonator 310 about its resonant frequency.The oscillations of the resonator 310 are detected by a detector 320,and fed back through the input 315 to enable the resonator to functionas a stable frequency source. At 330, a pilot signal close in frequencyto the resonant frequency of the resonator 310 is applied to theresonator, and the resonator entrains to that frequency withinmilliseconds, depending on how close the pilot frequency is to theresonant frequency, and the Q of the oscillator.

The signals applied to the resonator 310 cause a localized heating ofthe resonator, causing a deformation of the resonator. They may beapplied optically such as by laser, or thermally, such as by resistiveheating by microheaters, either directly on the resonator or proximatethereto. Detection of oscillation of the resonator may be obtained bylaser, using different forms of interferometry, or by other means, suchas piezo resistive mechanisms. Motion can also be produced and detectedthrough electrostatic (also called capacitive) transduction.

In one embodiment, the input 315 of the micromechanical resonator is a50Ω gold resistor, lithographically defined on the surface of theresonator, such as a dome or shell, which couples Joule heat tomechanical stress in the shell, inducing out-of-plane displacement. Thedome resonates when the frequency of the current flowing through theresistor, or microheater matches a resonant mode of the structure. Thedriving force provided by the thermal actuator is described by (1)

F_(ω) _(o) ∝V_(RF) ²/R∝V_(DC)V_(o) sin(ω_(o)t)/R ,  (1)

where ΔT is the local change in temperature and R is the resistance ofthe actuator.

The small thermal time constant of the thin film resonator allows thelocal temperature to be modulated and the heat dissipated at a ratecomparable to the time constant of mechanical motion at resonanceallowing driven resonance and high modulation rates. The resultingmotion is detected via detector 320, such as a HeNe laser using theresonator and sacrificial oxide cavity as a Fabry-Perot interferometer.Two resistors connected to independent bonding pads can be defined onthe resonator, allowing the possibility of multiple electricallyisolated resonator transducers. For two resistors spaced about 20 μmapart, −50 dB of electrical crosstalk may be measured.

By applying the laser detected displacement signal as feedback onto theinput transducer, the micromechanical dome resonator can function as astable frequency source. The photodetector signal of detector 320,representing the mechanical motion of the resonator, may be amplified byabout 50 dB by an amplifier 405 shown in FIG. 4, depending on theintensity of the detection laser. Other detection methods may be used,such as piezo, capacitive, optical or combinations thereof.

To select the resonator mode of vibration and to provide adjustablein-loop phase, the feedback signal may be filtered by a low-Q band passelement 315. A resonant frequency modifier 410 may be used to modify theresonant frequency of the resonator. In one embodiment, the modifier 410provides a DC bias less than 1 V, which is superimposed on the feedbacksignal and subsequently applied to the driving resistor. Limit-cycleoscillations at the free-running frequency, f_(FR), will grow out of theunstable equilibrium point of the system when the feedback network istuned to provide a gain greater than 1 with a phase shift of an integermultiple of 2π. The amplitude of the oscillations may be limited by thenonlinearity of either the mechanical resonator or the amplifier. Afrequency generator with short-term stability of 1.5 ppm may beachieved.

The resonant frequency, f_(o) of the dome and thus, f_(FR) of theoscillator, can be easily tuned by changing the amount of heatdissipated into the polysilicon film. Steady-state heat, imposed eitherby the HeNe detection laser or by a DC bias on the thermal actuator,will cause a change in resonator stiffness due to thermal expansion inthe film, changing the natural frequency of the shell resonator.Depending on the location of the heat source and the sensitivity of theeffective spring constant of a shell resonant mode to thermal expansion,a frequency deviation of 0.35% over a 1V change in DC bias can beachieved.

A weakly non-linear self-oscillatory system can be synchronized to aperiodic force superimposed on the system, provided that the naturalfrequency and the perturbation frequency are not far different. Limitcycle oscillations (in the absence of external forcing) in amicromechanical resonator may be locked in frequency and phase to asmall perturbation or pilot signal, f_(pilot), as provided at 330 inFIG. 3, which may be superimposed on the resonator via a modulatedlaser, resistor, or other means of transferring heat. The MEMSoscillator may be locked with a thermally induced pilot signal 330.

In one embodiment, positive feedback is applied via input 315, causingself-generation at frequency f_(FR). The pilot signal 330 is then usedto entrain the oscillator. It is applied to a second input heater on anisolated signal path. The pilot signal may be swept in frequency toestablish the region where the mechanical oscillator is entrained.Within the region of entrainment, the resonator oscillations take on thefrequency stability of the pilot signal. Hysteresis may be observedbetween the points where lock is lost (the pull out frequency) on theupward pilot sweep and where lock in resumed on the downward sweep. Theperturbation is then incremented in amplitude, which serves to broadenthe entrainment region.

Phase lag in the entrained MEMS resonator may be induced by detuning thefundamental frequency of the resonator, f_(o), with a DC bias via 410.Changing f_(o) moves the entrainment “V” relative to the pilot signal.To maintain frequency synchronization, the phase of the mechanicalvibrations changes according to the phase-frequency function of theresonator. This phase change can be measured between the pilot signaland the self-generation feedback signal. Thus, a phase difference can beobtained by simply changing the magnitude of the voltage impressed onthe oscillator rather than requiring a complex method of changing thepilot signal frequency. Furthermore, by changing f_(o) and notf_(pilot), the output phase can be tuned while f_(FR) remains unchanged.

FIG. 5 is a graph that illustrates that the total phase shift betweenf_(FR) and f_(pilot) can be controlled by at least 200°. Beyond thisregion of DC bias tuning, the region of the entrainment V may be shiftedto the extent that the oscillator may lose the lock on the pilot signal,and the entrained condition required to produce the phase differentialwill collapse, producing quasiperiodic motion. The data is for oneparticular geometrical configuration of resonator, and is notrepresentative of fundamental limits.

FIG. 6 is a block diagram of a phased array radar system 600 with phasemodifiable emitters according to an example embodiment. The phasemodulated resonator of FIG. 4 may be used to create an array ofindependently phase modulated elements having antennas, as indicated at610 in the phased array radar system 600. A feed network 620 is used tocouple each of the oscillators to an exciter 625 and emitter 630. Theseare coupled through a circulator 635, that allows signals to proceed inone direction. Thus, signals from the exciter 625 are fed to the array,while signals received from the array are fed to receiver 630. Eachphase modulated element 610 includes a variable resistor 640 andvariable phase element 645 selectively coupled via a switch 650 toeither a high power amplifier 655 for transmitting signals via theantennas, or receiving signals via a limiter 660 and low noise amplifier665. Each element 610 also includes a circulator 670 for properlydirecting signals to be sent and received. Such arrays of emittersprovide individually controlled emitter phase.

Constructive and destructive interference among the signals radiated bythe individual antennas determine the effective radiation pattern of thearray. FIGS. 7A, 7B, 7C and 7D are graphical examples of arrays ofemitters with different controlled phase changes over the array to shapeand control beams according to an example embodiment. FIG. 7Aillustrates the phase changing linearly across an array of emitters. Thearray is represented as a single row of emitters, but may actuallyconsist of a two dimensional array with varying phases. The phase of theemitters is represented by line 710 as linearly changing from left toright across the array. This results in a transmitted signal 715perpendicular to line 710. The signal 715 is directed toward the rightin FIG. 7A.

FIG. 7B illustrates an array of emitters wherein two separate linearlychanging phases 720 and 725 result in signals being transmitted bothleft 730 and right 735 respectively from a single array of emitters.FIG. 7C illustrates a further example of the array being split toprovide two sets of linear phases, 740 and 745, resulting in onetransmitted signal 750 being directed toward the left, and the othertransmitted signal 755 being directed straight out from the array. InFIG. 7D, two phases 760 and 765 result in signals being transmitted farleft 770, and slightly left 775.

Multiple different phase patterns may be obtained via the independentcontrol over each emitter in the compact package. The phases may bemodified real time to beam sculpt and allow the array to track or followmultiple objects moving in different directions. The use of suchindependently phase adjustable MEMS devices, whose phases arecontrollable without the use of switches or other moving parts, otherthan the oscillator itself, provides the ability to produce phased arrayradar systems in a small package, increasing the applications availablefor such systems.

In further embodiments, the adjustable phase MEMS devices may be used insonar and ultrasound applications in a similar manner. Further, acousticor radio frequency emitters may also be formed.

FIG. 8 is a block diagram of a system 800 for modulating the phase orfrequency of a micromechanical reference oscillator. Besides thepotential for delay line applications, the entrained oscillator can alsoproduce a phase or frequency modulated carrier signal for use incommunication systems. In the simplest case, frequency modulation of theoscillators' free running frequency can be achieved by applying abaseband AC signal 810 that will perturb the resonators fundamentalfrequency. The low-frequency bias causes f_(o) to change, creating acarrier frequency, f_(FR), which is modulated at the rate of thebaseband signal on the resistor. The depth of the modulationsuperimposed on the carrier is defined by the Hz/Volt transfer functionof the resistive actuator and resonator mode. To eliminate addercircuitry and achieve better isolation between the baseband signal andthe carrier signal, the baseband signal can be applied to a secondresistive actuator. In one embodiment, frequency modulation of a 26 MHzcarrier by a 30 kHz baseband signal may be used with a modulation depthof 15 kHz. A spectrum analyzer with a high video bandwidth, centered onthe carrier frequency, may be used to demodulate the carrier.

Phase modulation may be obtained by superimposing a baseband AC signalonto a pilot 815 microheater 817 while the resonator is self-generatingand entrained by the pilot signal 810. Positive feedback 820 from aphotodiode 825 is provided by a microheater 827, while the microheater817 is used to entrain the resonator with the pilot as well as supplythe AC baseband signal. The time varying baseband signal, through theadditional heat dissipated in the resistive actuator, pulls the naturalmechanical frequency across the pilot frequency. Detuning the resonatorcauses a time varying phase difference between the pilot signal and thefeedback signal that is proportional to the time varying basebandamplitude. The phase modulated carrier signal can be sampled from theoscillator feedback with an I/Q demodulator. Phase modulation of a 26MHz carrier by a 20 kHz baseband signal may be obtained with amodulation depth of 160°. Many different carrier frequencies andbaseband signals may be used in various embodiments.

CONCLUSION

Using the non-linear dynamics of a micron-sized mechanical oscillator, afrequency source may be provided with the ability to tune the phase andfrequency of the output signal. By controlling the intrinsic resonantfrequency of an injection locked micromechanical oscillator through avariety of methods, one can produce (depending on the implementation)continuous phase or frequency shifts in the output signal. An array ofsimilar resonators may be locked to a single, highly stable, frequencysource (common in the HF or VHF frequency range) and individuallydetuned with a separate signal. Upconversion to the EHF frequency rangefor radar applications may be done using a single source following themechanical phase tuning stage.

Silicon based MEMS tunable oscillators provide an alternative todiscrete components in communication architectures such as quartzcrystal frequency sources or voltage controlled oscillators. A high Q,tunable frequency source, such oscillators may be readily incorporatedinto standard integrated circuit fabrication processes, a critical steptoward the realization of integrated radio-on-chip communicationsystems. Such an implementation where both the frequency and the phaseof a micromechanical oscillator can be electronically continuouslycontrolled offers the possibility for highly integrated signalprocessing schemes.

The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow thereader to quickly ascertain the nature and gist of the technicaldisclosure. The Abstract is submitted with the understanding that itwill not be used to interpret or limit the scope or meaning of theclaims.

1. A phased array system comprising: an array of microelectromechanicalindependently phase modulated resonators; an exciter to drive theresonators in the array to resonate at their resonant frequencies,wherein each resonator is coupled to an antenna to transmit and receivesignals; and a receiver coupled to receive signals from the resonators.2. The system of claim 1 wherein the resonators are selected from thegroup consisting of a disc supported by a pillar, a cantilevered beamand a bridge.
 3. The system of claim land further comprising acirculator coupled between the array and the exciter and receiver tocontrol signal propagation direction.
 4. The system of claim 1 andfurther comprising a variable phase element coupled to each resonator.5. The system of claim 4 wherein the variable phase element is tocontrol phase change over the array to shape and control radar beams. 6.The system of claim 5 wherein the phase is controlled to change linearlyacross the array.
 7. The system of claim 5 wherein the phase iscontrolled to provide two sets of linear phases.
 8. The system of claim5 wherein the phase is controlled to follow objects moving in differentdirections.
 9. The system of claim 1 wherein each phase modulatedresonator includes a high power amplifier to provide signals fortransmitting via the antenna, and includes a low noise amplifier toamplify signals received from the antenna.
 10. The system of claim 9wherein each phase modulated resonator further includes a circulator tocontrol signal propagation direction to and from the antenna.
 11. Aphased array system comprising: an array of modules havingmicroelectromechanical independently phase modulated resonators, eachmodule including a phase modulating element, a high power amplifier toprovide signals to be transmitted from an antenna, and a low noiseamplifier to amplify signals received from the antenna; an excitercoupled to the modules via a feed network to drive the resonators in thearray to resonate at their resonant frequencies; and a receiver coupledto receive signals from the resonators.
 12. The system of claim 11wherein the phased array system is a phased array radar system.
 13. Thesystem of claim 11 wherein each module further includes a switch toselect between the low noise amplifier and the high power amplifier. 14.The system of claim 11 wherein the resonators are selected from thegroup consisting of a disc supported by a pillar, a cantilevered beamand a bridge.
 15. The system of claim 4 wherein the variable phaseelement is to control phase change over the array to shape and controlradar beams.
 16. The system of claim 5 wherein the phase is controlledto change linearly across at least a portion of the array.
 17. A methodcomprising: coupling an exciter to an array of microelectromechanicalindependently phase modulated resonators to drive the resonators in thearray to resonate at their resonant frequencies; controlling phasechanges over the array of resonators; transmitting signals from theresonators via an antenna to shape and control beams as a function ofthe phase changes; and receiving signals via the antenna.
 18. The methodof claim 17 wherein the phase is controlled to change linearly acrossthe array.
 19. The method of claim 17 wherein the phase is controlled toprovide two sets of linear phases.
 20. The method of claim 17 whereinthe phase is controlled to follow objects moving in differentdirections.